The invention relates to a heat sealing device useful for making pouches filled with flowable materials, e.g. thick sauces and dressings, particularly to pouches made on so-called vertical form, fill and seal machines.
It is well known to package flowable materials, for example, milk, on so-called vertical form, fill and seal machines. Using one such a machine, a flat web of thermoplastic film is unwound from a roll and formed into a continuous tube in a tube forming section, by sealing the longitudinal edges of the film together to form a so-called lap seal or a so-called fin seal. The film tube thus formed is pulled vertically downwards to a filling station. The tube is then collapsed across a transverse cross-section of the tube, the position of the cross-section being at a sealing device below the filling station. A transverse heat seal is made, by the sealing device, at the collapsed portion of the tube, thus making an airtight seal across the tube. The sealing device generally comprises a pair of jaws. After the jaws are closed and while the transverse seal is being made, a quantity of material to be packaged, e.g. liquid, is caused to enter the film tube, at the filling station, and fill the tube upwardly from the aforementioned transverse seal. When the jaws are opened the film tube is then caused to move downwardly a predetermined distance. Such movement may be under the influence of the weight of the material in the tube, or may be caused by pulling or mechanically driving the tube. The jaws of the sealing device are closed again, thus collapsing the tube at a second transverse section. The second transverse section may be above, usually just above, the air/material interface in the tube, or the second transverse section may be below the air/material interface. The sealing device clamps, seals and severs the tube transversely at the second transverse section. The material-filled portion of the tube is now in the form of a pillow shaped pouch. Thus the sealing device has sealed the top of the filled pouch, sealed the bottom of the next-to-be formed pouch, all in one operation. Contamination of the heat sealing surfaces of the tube may occur due to splashing or drooling of the material to be packaged. One such vertical form and fill machine of the type described above is sold under the trade mark PREPAC. Other machines may be operated such that the material-to-be-packaged is caused to enter the tube continuously rather than intermittently as described above. As a result, sealing of the film takes place with material-to-be-packaged between the heat sealing surfaces in the tube.
With some other machines, the sealing device does not sever the tube at the second transverse section, but does sever the tube subsequently. With yet other machines the jaws of the heat sealing device reciprocate up and down. With such machines, the jaws clamp, seal and sever the tube of film while moving in a downward direction, the jaws then open and return upwards in the open position. The downward movement of the closed jaws also serve to advance the tubular film downwardly.
For many years, milk has been packaged in pouches made on vertical form and fill machines. Such pouches have been sold to household consumers and, in use, such milk-filled pouches are stood within an open-mouthed pitcher. More recently, such pouches have been used to package other flowable comestibles, mayonnaise, caramel, scrambled eggs, tomato ketchup, chocolate fudge, salad dressings, preserves and the like. Pouches containing such comestibles are usually sold to "institutional" buyers, e.g. restaurants.
A sealing device commonly used is a so-called "impulse sealer" in which an electrical current flows through the sealing element for only a fraction of the cycle time between operations. The impulse sealer may be a round wire, e.g. a "piano" wire about 2.0 mm to 2.3 mm diameter, electrically insulated from a water-cooled supporting jaw. After the electrical impulse is fed to the impulse sealer, there is a cooling period in which the seals are "set" while the jaws remain closed. Round wire impulse sealers as described above, in combination with conventional flat faced heat sealing jaws, are satisfactory for form and fill machines when packaging liquids such as milk, water or other highly aqueous products. These liquids have relatively high thermal conductivities and thus aid in cooling of the seal after the sealing jaws are opened, i.e. the cooling effect of the liquid on the seal operates before the weight of the liquid can weaken or rupture the bottom seal. Sometimes, too, such liquids vaporize at temperatures below the sealing temperature of the sealing layer of the film and it is speculated that vaporization may aid in removing contamination from the seal area, i.e. the liquid may be "self voiding" from the seal area. Round wire impulse sealers are generally unsatisfactory on form, fill and seal machines when packaging thick flowable materials such as mayonnaise, chocolate fudge, scrambled egg mix, dressings, jams and the like. Other kinds of sealers are known, e.g. as disclosed in U.S. Pat. No. 3,692,613, which issued 1972 Sep. 19 to R. E. Pederson, in U.S. Pat. No. 4,115,182, which issued 1978 Sep. 19 to M. M. Wildmoser and U.S. Pat. No. 4,744,845, which issued 1988 May 17 to J. Posey. Heretofore, however, arrangements of heat sealing elements and heat sealing jaws have not been entirely satisfactory for sealing through thick flowable materials, especially those which contain high sugar concentrations, e.g. above about 50%, or high fat concentrations, e.g. above about 5%. One such material containing high concentrations of sugar and fat is chocolate fudge sundae topping. When sealing through materials it is important to void, as much as possible, the material from the seal area.
Other shapes of impulse heat sealing elements are known. For example, one heat sealing element is made from a flat strip of metal upon which is spot welded a round wire. Another is made from a flat strip of metal, bent so that the heat sealing element comprises two flat elongated fins adjoined by a segment of a tube, each lateral edge of said segment being adjoined to a lateral edge of one of the elongated fins. Such heat sealing elements are intended to provide sufficient pressure and heat at the wire or part-tubular portion to sever the flattened pouch and to form a heat sealed bead at the severed edge, and the angled flat strip/fins provide less pressure than at the tubular part and a decreasing pressure as a function of distance from the tubular part. This pressure and heat provides a wider seal band on the heat sealed film and assists in voiding the flowable material from the heat seal area.
It has been found that on vertical form, fill and seal machines with conventional heat seal assemblies excessive tension on either side of the transverse seal, while it is in a molten or semi-molten state, can elongate the seal and result in weak seals or in so-called "leakers" This can be caused by the weight of the filled pouch pulling down on the molten or semi-molten seal from beneath the jaw, i.e. on the top seal of the pouch. It can also be caused by the film pulling away from the molten or semi-molten seal on the upper side of the jaw, i.e. on the bottom seal of the pouch, due to tension in the film as the pouch is being filled with flowable material. Attempts have been made to overcome such problems. For example 1st Addition to French Patent No. 1 445 557, No. 91 657, granted 1968 Jun. 17 to Thimonnier & Co., Lte, discloses means to grip the tubular film above and below the heat seal jaw. Such means are rigid and do not relieve the aforementioned tension to any substantial degree. The present invention seeks to improve the invention of Thimonnier and to minimize such problems.
Accordingly, the present invention provides a heat sealing assembly, for sealing at least two layers of thermoplastic film, comprising two jaws, resilient clamping elements and a heat sealing element, at least one of the jaws being capable of transverse motion and adapted to collapse a tubular film made from the thermoplastic film and passing between the jaws, each of said jaws having at least one of said elements mounted thereon, said clamping elements being adapted to grip thermoplastic film which passes therebetween and urge said gripped film towards said jaws when the jaws close and to release said film when the jaws open.
In one embodiment the heat sealing element is mounted on a first jaw and a resilient heat seal back-up pad is mounted on a second jaw, and a pair of clamping elements are mounted separately on one side of said jaws and are adapted i) to synchronously move with said jaws, ii) clamp film between said clamping elements when the jaws close and iii) cooperate in urging the clamped film towards the closed jaws.
In another embodiment there is one pair of resilient clamping elements on one side of the jaws and another pair of resilient clamping elements on the other side of the jaws.
In a further embodiment the clamping elements are made of rubber.
In yet another embodiment the resilient material has a thickness such that the ends of the cooperating portions are constructed such that the ends slope away from one another towards the part of the thickness furthest away from the jaws.
The invention also provides a process for slackening a tube of thermoplastic film in the immediate area of a transverse heat sealing jaw just before, during and just after the heat sealing jaws of the assembly are closed to form the transverse seal, said process comprising:
i) causing the tube to be advanced past a heat sealing assembly which comprises two jaws, at least one pair of resilient clamping elements and a heat sealing element, at least one of the jaws being capable of transverse motion and adapted to collapse the tube, one of said jaws having said heat sealing element mounted thereon,
ii) gripping said tube with said clamping elements synchronous with the jaws moving towards each other and urging the gripped tube towards said heat sealing element with the clamping elements, such that the tube is slack in the area between the clamping elements and the heat sealing element,
iii) holding the tube with the clamping elements while the tube is being gripped by the jaws and transversely sealed with the heat sealing element, and
iv) releasing the tube from being gripped by the jaws and the clamping elements.
In one embodiment the tube is gripped by a pair of clamping elements mounted separately on one side of said jaws.
In yet another embodiment the tube is gripped by one pair of clamping elements on one side of the jaws and another pair of clamping elements on the other side of the jaws.
In yet another embodiment the tube is gripped by clamping elements which has a thickness such that the ends of the cooperating portions are constructed such that the ends slope away from one another towards the part of the thickness furthest away from the jaws.